Implantable pacemakers and cardioverter defibrillators (ICDs) are available for delivering electrical stimulation therapies to a patient's heart, such as bradycardia pacing, cardiac resynchronization therapy (CRT), anti-tachycardia pacing and cardioversion/defibrillation shocks. Medical device technology advancement has led toward smaller and smaller implantable devices. Recently, cardiac pacemakers have been introduced which can be implanted directly in a heart chamber. In some examples, such pacemakers may be leadless and delivered into the heart chamber using a catheter. Such miniaturized pacemakers may be referred to as intracardiac pacing devices (PDs), although they may be epicardially or extracardially implanted, in some examples. The introduction of such PDs, and the resulting elimination of the need for transvenous intracardiac leads, provides several advantages. For example, complications due to infection associated with a lead extending from a subcutaneous pacemaker pocket transvenously into the heart can be eliminated. Other complications, such as “twiddler's syndrome,” lead fracture, or poor connection of the lead to the pacemaker are eliminated in the use of an intracardiac PD.
Various technologies are directed to controlling an intracardiac PD to deliver pacing pulses in synchrony with paced or sensed events occurring in other heart chambers. Cardiac resynchronization therapy (CRT) is an example of a pacing therapy that includes delivering pacing pulses in a heart chamber at a predetermined time interval after a sensed or paced event in another heart chamber. CRT is a treatment for heart failure patients in whom one or more heart chambers are electrically paced to restore or improve heart chamber synchrony. Improved heart chamber synchrony is expected to alleviate symptoms of heart failure.
Achieving a positive clinical benefit from CRT, however, may be dependent on several therapy control parameters, such as the timing intervals used to control pacing pulse delivery to one or both ventricles, e.g., an atrio-ventricular (AV) interval and/or an inter-ventricular (VV) interval. The AV interval controls the timing of ventricular pacing pulses relative to a preceding atrial depolarization, intrinsic or paced. The VV interval controls the timing of a pacing pulse in one ventricle relative to a paced or intrinsic sensed event in the other ventricle. Pacing may be delivered in the right ventricle (RV) and/or the left ventricle (LV) to restore ventricular synchrony.
CRT includes delivering pacing stimuli to both ventricles, or to one ventricle with the intended result of a substantially simultaneous mechanical contraction and ejection of blood from the ventricles. Ideally, each pacing pulse stimulus delivered to a ventricle would evoke a response from the stimulated ventricle. In order to evoke the desired response, it is preferable to time the delivery of the ventricular pacing so as to be delivered at a target point in time subsequent to a P-wave of a cardiac cycle of the patient. If the ventricular pacing signal is delivered too late after a P-wave, then the pacing therapy may potentially coincide with the occurrence of an R-wave of the cardiac cycle of the patient.